1. Field of the Invention
The present invention generally relates to power supply control devices, power supply circuits, power supply control methods, and electronic apparatuses. More particularly, the present invention relates to a power supply control device, a power supply circuit, a power supply control method, and an electronic apparatus, in which current to be supplied to a load is controlled by a transistor disposed between a power source and the load.
2. Description of the Related Art
In recent years, portable electronic apparatuses have been widespread. A portable electronic apparatus operates with a battery unit as its power source. Accordingly, the operating time of the apparatus by the battery unit is an important factor in measuring the performance of the apparatus. Such a portable electronic apparatus does not drive an internal circuit with the voltage of the battery unit. Therefore, a power supply circuit that converts the voltage of the battery unit into a voltage suitable for the internal circuit is contained in the apparatus.
It is preferable that the power consumption of the apparatus be reduced so as to prolong the operation time, while high efficiency is maintained in the power supply circuit. However, a high-efficient power supply circuit consumes a large amount of power.
Conventionally, synchronous DC-DC converters are used to improve the efficiency of a power supply circuit. As the performance of the CPUs contained in electronic apparatuses has been increased, the power consumption has been becoming larger. To restrict such an increase in power consumption, the voltage used in an electronic apparatus has been decreasing. As a result, the output of a DC-DC converter has become low-voltage, large-current output.
In view of this, it is essential to protect a DC-DC converter when short-circuiting or overloading occurs. Example methods of protecting a DC-DC converter from short-circuiting and overloading includes: a constant current control method in which the maximum value of the output current of the DC-DC converter is restricted through monitoring the output current; and an excess current preventing circuit method in which, the instant an excess current is detected, the DC-DC converter is stopped.
Generally, a current sense resistor is disposed in the output circuit of the DC-DC converter so as to monitor the voltage caused by the current flowing through such a current sense resistor. In a portable electronic apparatus, a battery unit is used as a power source for the internal devices. As a battery keeps discharging, the voltage of the battery normally drops. Therefore, DC-DC converters are employed to maintain the voltage used in an electronic apparatus at a constant value.
In a notebook-type computer, for instance, various devices such as a semiconductor device, a storage device, and a display device are mounted. These devices operate at different voltages from each other. More specifically, devices such as a HDD, CD-ROM, and DVD operate at 5.0 V, while a memory and a semiconductor device for controlling peripheral circuits operate at 3.3 V, for instance. As for the CPU, the operating voltage is 0.9 to 2.0 V.
Meanwhile, power is supplied to such a notebook-type computer from an external power source such as an AC adapter, or from a battery unit contained in the apparatus. In this case, a DC-DC converter is employed to produce various voltages required by the devices in the apparatus.
For such a DC-DC converter, a switching regulator type is widely used for its high efficiency. In a DC-DC converter of the switching regulator type, a transistor is disposed between a power source and a load, and this transistor is controlled so as to control the output voltage. To detect the current to be supplied to the load, a current sense resistor is connected in series between the transistor and the load. As the current to be supplied to the load flows through the current sense resistor, a voltage corresponding to the current is generated across the current sense resistor. By detecting this voltage generated across the current sense resistor, the current to be supplied to the load can be detected. Using the detected current, a control operation such as an excess current preventing operation is performed.
FIG. 1 is a block diagram of a power system of an electronic apparatus.
In a portable electronic apparatus 1, such as a notebook computer, a commercial AC power supply 3 is converted into a DC power supply by an AC adapter 2, and the DC power supply is generally used as the driving power. The conversion is externally carried out. The electronic apparatus 1 comprises a battery unit 5, a charger 6, diodes D11 and D12, and DC-DC converters 7-1 to 7-3. With this structure, the DC power from the AC adapter 2 is supplied to internal units 4-1 to 4-3.
The battery unit 5 is used as the driving power when the electronic apparatus 1 is being carried. The charger 6 recharges the battery unit 5 with external power from the AC adapter 2.
The diode D11 prevents power supply from the battery unit 5 to the AC adapter 2. The diode D12 prevents direct voltage application from the AC adapter 2 to the battery unit while the AC adapter 2 is connected to the electronic apparatus 1.
The DC-DC converter 7-1 converts DC voltage from the AC adapter 2 or the battery unit 5 into DC voltage demanded by the internal unit 4-1, and then supplies the converted DC voltage to the internal unit 4-1. The DC-DC converter 7-2 converts DC voltage from the AC adapter 2 or the battery unit 5 into DC voltage demanded by the internal unit 4-2, and then supplies the converted DC voltage to the internal unit 4-2. The DC-DC converter 7-3 converts DC voltage from the AC adapter 2 or the battery unit 5 into DC voltage demanded by the internal unit 4-3, and then supplies the converted DC voltage to the internal unit 4-3.
FIG. 2 is a block diagram of conventional DC-DC converters.
The DC-DC converters 7-1 to 7-3 each comprise a power supply control IC 10, a main switching transistor Tr1, a synchronous rectifying transistor Tr2, diodes D1 and D2, a choke coil L1, a smoothing capacitor C1, a back-flow preventing capacitor C2, and a current sense resistor R1.
An input voltage Vin is supplied to an input terminal Tin. The input terminal Tin is connected to the power supply terminal Tvin of the power supply control IC 10 and the drain of the main switching transistor Tr1.
The main switching transistor Tr1 is constituted by an n-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The drain of the main switching transistor Tr1 is connected to the input terminal Tin, while the source is connected to an output terminal Tout via the choke coil L1 and the current sense resistor R1. The gate of the main switching transistor Tr1 is connected to a terminal Tdh of the power supply control IC 10. The main switching transistor Tr1 is switched on and off, depending on a pulse supplied from the terminal Tdh of the power supply control IC 10.
The output current of the main switching transistor Tr1 is supplied to the choke coil L1. The choke coil L1, the synchronous commutating transistor Tr2, and the diode D2 constitute a rectifier circuit that rectifies the pulse-type output current from the main switching transistor Tr1.
The anode of the diode D1 is grounded, and the cathode of the diode D1 is connected to the choke coil L1. The diode D1 is a flywheel diode that supplies forward current to the choke coil L1 when the main switching transistor Tr1 is switched off, and is provided with reverse voltage and switched off when the main switching transistor Tr1 is on.
The synchronous rectifying transistor Tr2 is constituted by an n-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The drain of the synchronous rectifying transistor Tr2 is connected to the source of the main switching transistor Tr1, while the source thereof is grounded. The gate of the synchronous rectifying transistor Tr2 is connected to a terminal Td1 of the power supply control IC 10. The synchronous rectifying transistor Tr2 is switched off by the power supply control IC 10 when the main switching transistor Tr1 is switched on, and is switched on by the power supply control IC 10 when the main switching transistor Tr1 is switched off. The synchronous rectifying transistor Tr2 is switched on when the forward current flows through the diode D1, so as to reduce a voltage decrease due to the forward voltage of the diode D1.
The current rectified by the choke coil L1, the diode D1, and the synchronous transistor Tr2 is supplied to the output terminal Tout via the resistance R1 of the current sense resistor R1. The connection point between the choke coil L1 and the current sense resistor R1 is connected to a terminal Tcs of the power supply control IC 10.
The output terminal Tout is connected to the ground via the smoothing capacitor C1, as well as to a terminal Tfb of the power supply control IC 10. The smoothing capacitor C1 smoothens current supplies from the choke coil L1 via the current sense resistor R1.
Furthermore, a driving power terminal Tvg is connected to a terminal Tvb of the power supply control IC 10. A gate driving voltage VG is applied to the driving power terminal Tvg. A terminal Tcb of the power supply control IC 10 is connected to the connection point between the diode D2 and the back-flow preventing capacitor C2. The anode of the diode D2 is connected to the driving power terminal Tvg, while the cathode thereof is connected to the back-flow preventing capacitor C2. The diode D2 prevents the current from flowing back toward the terminal Tvg when the voltage of a terminal Tcb is increased by the back-flow preventing capacitor C2. One end of the back-flow preventing capacitor C2 is connected to the connection point between the cathode terminal of the diode D2 and the terminal Tcb, while the other end thereof is connected to the connection point of the source of the main switching transistor Tr1, the drain of the synchronous rectifying transistor Tr2, the cathode of the diode D1, and one end of the choke coil L1.
Referring now to FIG. 3, the power supply control IC 10 will be described in greater detail.
The power supply control IC 10 comprises a differential amplifier 11 (AMP1), error amplifiers 12 and 13 (ERA2 and ERA1), a saw-tooth oscillator 14, a PWM, (Pulse Width Modulation) comparator 15, drive amplifiers 16 and 17 (DRV1 and DRV2), reference voltage source 18 and 19, and resistors R2 and R3.
The non-inverting input terminal of the differential amplifier 11 is connected to the terminal Tcs, while the inverting input terminal thereof is connected to the terminal Tfb. Accordingly, the differential amplifier 11 outputs a differential signal in accordance with the potential difference between the terminal Tcs and the terminal Tfb. The terminal Tcs and the terminal Tfb are connected to both ends of the current sense resistor R1 outside the power supply control IC 10. Accordingly, the differential signal outputted from the differential amplifier 11 depends on the current flowing through the current sense resistor R1.
The differential signal outputted from the differential amplifier 11 is supplied to the inverting input terminal of the error amplifier 12. A reference voltage e2 generated from the reference voltage source 18 is applied to the non-inverting input terminal of the error amplifier 12. The error amplifier outputs a signal that varies depending on the difference between the differential signal outputted from the differential amplifier 11 and the reference voltage e2. Accordingly, the output of the error amplifier 12 is small when the current flowing through the current sense resistor R1 is great. On the other hand, the output of the error amplifier 12 is great when the current flowing through the current sense resistor R1 is small. The output signal of the error amplifier 12 is supplied to a non-inverting input terminal of the PWM comparator 15.
A divided voltage between the resistors R2 and R3 is supplied to the inverting input terminal of the error amplifier 13, while a reference voltage e1 of the reference voltage source 19 is supplied to the non-inverting input terminal. The resistors R2 and R3 are connected in series between the terminal Tfb and the terminal Tgnd, so as to divide the voltage applied to the terminal Tfb. The terminal Tfb is connected to the output terminal Tout outside the power supply control IC 10. The resistors R2 and R3 divide the output voltage Vout, and supply it to the error amplifier 13.
The error amplifier 13 outputs a value that is obtained by subtracting the divided voltage between the resistors R2 and R3 from the reference voltage e1 generated from the reference voltage source 19. When the output voltage Vout of the output terminal Tout is small, the output of the error amplifier 13 is great. On the other hand, when the output voltage Vout of the output terminal Tout is great, the output of the error amplifier 13 is small. The output of the error amplifier 13 is supplied to a non-inverting input terminal of the PWM comparator 15.
A saw-tooth signal is supplied from the saw-tooth oscillator 14 to an inverting input terminal of the PWM comparator 15. The PWM comparator 15 compares the saw-tooth signal with the signals outputted from the error amplifiers 12 and 13, so as to output a pulse in accordance with the comparison result. The PWM comparator 15 compares one of the outputs of the error amplifier 12 and 13, whichever is smaller, with the saw-tooth wave generated from the saw-tooth oscillator 14. The PWM comparator then outputs a pulse that is high when the smaller output between the error amplifiers 12 and 13 is smaller than the saw-tooth wave, and low when the smaller output between the error amplifiers 12 and 13 is greater than the saw-tooth wave.
FIG. 4 is an operation waveform chart of a conventional power supply control IC when performing voltage control. FIG. 5 is an operation waveform chart of another conventional power supply control IC when performing current control. In FIGS. 4 and 5, (A) shows the output waveforms of the error amplifiers 12 and 13, and the saw-tooth oscillator 14, and (B) shows the output waveform of the PWM comparator 15.
When the output current Iout is relatively small and the output voltage Vout is high, the output ERA1 of the error amplifier 13 and the saw-tooth wave are compared, as shown in FIG. 4A. The pulse outputted from the PWM comparator 15 varies in duty ratio of the high level to the low level, depending on the output ERA1 of the error amplifier 13, as shown in FIG. 4B. When the output voltage Vout is high, the pulse outputted from the PWM comparator 15 has a smaller pulse width at the high level and a greater pulse width at the low level. When the output voltage Vout is low, the pulse outputted from the PWM comparator 15 has a greater pulse width at the high level and has a smaller pulse width at the low level.
When the output voltage Vout is relatively low and the output current Iout is great, the output ERA2 of the error amplifier 12 and the saw-tooth wave are compared, as shown in FIG. 5A. The pulse outputted from the PWM comparator 15 varies in duty ratio of the high level to the low level, depending on the output ERA2 of the error amplifier 12, as shown in FIG. 5B. When the output current Iout is great, the pulse outputted from the PWM comparator 15 has a smaller pulse width at the high level and has a greater pulse width at the low level. When the output current Iout is small, the pulse outputted from the PWM comparator 15 has a greater pulse width at the high level and has a smaller pulse width at the low level.
The PWM comparator 15 supplies its output pulse to the drive amplifier 16. The drive amplifier 16 is connected to the terminal Tcb and the terminal Tv1, and is driven in accordance with the potential difference between the terminal Tcb and the terminal Tv1. The drive amplifier 16 generates a driving signal from the output pulse of the PWM comparator 15. The driving signal is used for driving the main switching transistor Tr1. The output of the drive amplifier 16 is outputted through the terminal Tdh. The terminal Tdh of the power supply control IC 10 is connected to the gate of the main switching transistor Tr1, so that a pulse that depends on the output pulse of the PWM comparator 15 can be supplied from the terminal Tdh to the gate of the main switching transistor Tr1.
The main switching transistor Tr1 is switched on when the pulse supplied from the terminal Tdh is high, and is switched off when the pulse supplied from the terminal Tdh is low.
Aside from the output pulse, the PWM comparator 15 also outputs an inverted output pulse that is obtained by inverting the output pulse. The inverted output pulse is supplied to the drive amplifier 17. The drive amplifier 17 is connected to the terminal Tvb, and is driven by the driving voltage VG supplied to the terminal Tvb. From the inverted output pulse, the drive amplifier 17 generates a driving signal for driving the synchronous rectifying transistor Tr2. The output of the drive amplifier 17 is outputted through the terminal Td1 of the power supply control IC 10. The terminal Td1 of the power supply control IC 10 is connected to the gate of the synchronous rectifying transistor Tr2, so that a pulse that depends on the inverted output pulse can be supplied to the gate of the synchronous rectifying transistor Tr2. The synchronous rectifying transistor Tr2 is switched on when the pulse supplied from the terminal Td1 is high, and is switched off when the pulse supplied from the terminal Td1 is low.
The pulse supplied to the gate of the synchronous rectifying transistor Tr2 is equal to a pulse obtained by inverting the pulse supplied to the gate of the main switching transistor Tr1. Accordingly, when the main switching transistor Tr1 is in the ON state, the synchronous rectifying transistor Tr2 is in the OFF state. On the other hand, when the main switching transistor Tr1 is in the OFF state, the synchronous rectifying transistor Tr2 is in the ON state.
In the above power supply control IC 10, the following relationship is satisfied:
Vout=[Ton/(Ton+Toff)]xc3x97Vin=(Ton/T0)xc3x97Vin
wherein Vout is the output voltage, Ton is the duration of the main switching transistor in the ON state, Toff is the duration of the main switching transistor Tr1 in the OFF state, and (Ton+Toff)=T0.
The following relationship is also satisfied:
xe2x80x83Iin=(Ton/T0)xc3x97Iout
wherein Iin is the average input current, and Iout is the output current.
In accordance with the above expressions, the output voltage Vout and the output current Iout can be constantly controlled by controlling the duty ratio in the main switching transistor Tr1 with the power supply control IC 10.
As described so far, the power supply control IC 10 having the above structure measures the output voltage Vout and the output current Iout. In accordance with the comparison between the output voltage Vout and the output current Iout, the main switching transistor Tr1 and the synchronous rectifying transistor Tr2 are controlled. However, the control operation is performed in accordance with the output voltage Vout, and power supply control ICs that perform a current control operation in an excess current state.
FIG. 6 is a block diagram of another conventional power supply control IC. In this figure, the same components as in FIG. 3 are denoted by the same reference numerals, and an explanation for those components is omitted.
A power supply control IC 21 comprises a comparator 22, a reference voltage source 23, a flip-flop 24, an OR gate 25, and an AND gate 26, in place of the differential amplifier 11, the error amplifier 12, and the reference voltage source 18 of FIG. 3.
The non-inverting input terminal of the comparator 22 is connected to the terminal Tcs, while the inverting input terminal thereof is connected to the terminal Tfb via the reference voltage source 23. The reference voltage source 23 generates a reference voltage e11. When the voltage at either end of the current sense resistor R1 is higher than the reference voltage e11, the comparator 22 outputs a high-level signal. On the other hand, when the voltage at either end of the current sense resistor R1 is lower than the reference voltage e11, the comparator 22 outputs a low-level signal. The flip-flop 24 is an R-S flip-flop. The output of the comparator 22 is supplied to the set terminal S of the flip-flop 24, and the saw-tooth wave outputted from the saw-tooth oscillator 14 is supplied to the reset terminal R of the flip-flop 24.
When the output of the comparator 22 becomes high, the flip-flop 24 is set. When the saw-tooth wave outputted from the saw-tooth oscillator 14 reaches a predetermined level, the flip-flop 24 is reset. Once the flip-flop 24 is set, it keeps outputting a high-level signal until it is reset. The flip-flop 24 outputs a non-inverted output Q and an inverted output *Q. The non-inverted output Q is supplied to the OR gate 25, while the inverted output *Q is supplied to the AND gate 26.
The non-inverted output Q of the flip-flop 24 and the inverted output of the PWM comparator 15 are supplied to the OR gate 25. The OR gate 25 performs an OR logic operation on the non-inverted output Q of the flip-flop 24 and the inverted output of the PWM comparator 15. The output signal of the OR gate 25 is supplied to the drive amplifier 17.
Meanwhile, the inverted output *Q of the flip-flop 24 and the non-inverted output of the PWM comparator 15 are supplied to the AND gate 26. The AND gate 26 performs an AND logic operation on the inverted output *Q of the flip-flop 24 and the non-inverting output of the PWM comparator 15. The output signal of the AND gate 26 is supplied to the drive amplifier 16.
When the voltage generated at either end of the current sense resistor R1 is higher than the reference voltage e11, the flip-flop 24 is set. When the flip-flop 24 is set, the non-inverted output of the flip-flop 24 becomes high, and the non-inverted output thereof becomes low.
When the non-inverted output of the flip-flop 24 becomes high, the output of the OR gate 25 also becomes high, regardless of whether the inverted output of the PWM comparator 15 is high or low. Accordingly, the output of the terminal Td1 remains high. While the output of the terminal Td1 is high, the synchronous rectifying transistor Tr2 remains in the ON state.
When the inverted output of the flip-flop 24 becomes low, the output of the AND gate 26 also becomes low, regardless of whether the non-inverted output of the PWM comparator 15 is high or low. Accordingly, the output of the terminal Tdh remains low. While the output of the terminal Tdh is low, the main switching transistor Tr1 remains in the OFF state. In this manner, the output current Iout is restricted so as to eliminate excess current.
When the output of the saw-tooth oscillator 14 reaches a predetermined level, the flip-flop 24 is reset. If an excess current has been eliminated at this point, the flip-flop 24 remains in the reset state, and the regular voltage control is performed. On the other hand, if an excess current has not been eliminated at this point, the flip-flop 24 is set again, and the main switching transistor Tr1 remains in the OFF state, thereby continuing the excess current eliminating operation.
In this structure, the main switching transistor Tr1 is switched off when excess current is detected, and the voltage control operation is performed when the excess current is eliminated. However, once the main switching transistor Tr1 is switched off when excess current is detected, it may remain in the OFF state until power supply is resumed, even if the excess current is eliminated.
FIG. 7 is a block diagram of yet another conventional power supply control IC. In this figure, the same components as in FIG. 6 are denoted by the same reference numerals.
A power supply control IC 31 is characterized by the flip-flop 24 that is not reset by the saw-tooth wave generated from the saw-tooth oscillator 14.
In any of the conventional power supply control ICs described so far, the current sense resistor is connected in series between the main switching transistor and a load. As a result, there is a problem of power loss caused by the current sense resistor. Such power loss becomes larger as the output current becomes greater.
To reduce power loss caused by the current sense resistor, the current sense resistor should be made smaller. However, even if the resistance value of the current sense resistor is very small, the power loss cannot be completely avoided. Also, a resistor having a small resistance value is costly. Furthermore, the current sense resistor is normally formed by a discrete component, which takes up considerable space.
A general object of the present invention is to provide power supply control devices in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a power supply control device, a power supply circuit, a power supply control method, and an electronic apparatus that are not costly, do not take up too much space, and can reduce power loss.
The above objects of the present invention are achieved by a power supply control device which determines whether or not a switching element is in an ON state, detects a voltage generated across the switching element in accordance with a current flowing through the switching element when the switching element is in the ON state, and controls the switching element in accordance with the detected voltage, thereby controlling an output voltage.
The above objects of the present invention are also achieved by a power supply circuit, a power supply control method, or an electronic apparatus, which employs the above power supply control device.
In this power supply control device, whether or not the switching element is in the ON state is determined in accordance with a control signal for controlling the switching element. Alternatively, whether or not the switching element is in the ON state is determined in accordance with a result of a comparison between the gate-source voltage of the switching element and a reference voltage. In yet another embodiment, the potential difference between the output voltage and a first reference voltage is detected, and whether or not the switching element is in the ON state is determined in accordance with a result of a comparison between the detected potential difference and a second reference voltage. The second reference voltage can be set based on an externally supplied voltage.
In accordance with the present invention, an output current can be detected using the ON-resistance of the switching element, thereby eliminating the need for a current sense resistor and power loss due to such a current sense resistor. Thus, the power consumption and costs can be reduced.
Furthermore, the voltage for detecting an ON state is set at a suitable value by varying the external voltage, so as to determine whether or not the switching element is certainly in the ON state.